a. Field of the Invention
The present invention relates to an apparatus for measuring small changes in the level of liquids stored in underground or aboveground tanks, which apparatus can be used as part of a system to detect leaks.
b. Brief Discussion of the Prior Art
Most methods used to detect small leaks in underground or aboveground storage tanks containing petroleum liquids or other chemical liquids considered hazardous to the environment use (1) a level sensor to estimate the changes in the volume of liquid that occur during a test and (2) a means to compensate for the volume changes that occur as a result of thermal expansion or contraction of the liquid. Most methods use a vertical array of temperature sensors to make the measurements that will be used to compensate for these thermally induced volume changes. In tests conducted on partially filled underground tanks, or those conducted on aboveground tanks, even large volume changes produce only small level changes, because the cross-sectional area of the liquid surface in these tanks is very large. In a half-filled 10,000-gal underground storage tank, for example, a volume change of 0.1 gal results in a level change of 0.00075 in. As a consequence, a level sensor with a high degree of precision is required for such tests.
A common method for measuring the change in level of a liquid in a storage tank is to measure the vertical displacement of a float resting on the surface of the liquid. There are many methods used to measure small liquid-level changes with a float system. One of the most common is shown in FIG. 1. In FIG. 1a, a displacement sensor 10 is mounted rigidly by means of a connector 11 in the fill tube 2 of a tank 1, and in FIG. 1b this sensor 10 is attached by means of a connector 12 to a mount 3 that extends downward from the fill tube 2. A rod 5 capable of vertical movement extends through the sensor, guided by means of a track, holder, or "bumper guide" 7. A float 6 resting on the surface of the liquid is affixed to the lower end of the rod 5. It is assumed that as the level of the liquid in the tank changes, the level of the float-and-rod subsystem 4 will change by an identical amount; this is true if the density of the liquid surrounding the float 6 does not change during the measurement period. The vertical displacement of the float 6 is measured by the rod's 5 movement through the sensor 10. In some systems, a portion of the rod 5 has calibrated markings that are read by the displacement sensor 10. There are many types of commercially available displacement sensors 10 that can be used to measure the movement of the rod 5. Strain gauges, optical, capacitance, electromagnetic, and acoustic sensors are some examples.
The accuracy of measuring level changes with the scheme illustrated in FIG. 1 depends on the frictional effects between the bumper guide 7 and the rod 5. Any contact between them may cause the rod 5 to stick. Contact is likely to occur because the center of gravity of the float-and-rod subsystem 4 is located above its center of buoyancy, making it rotationally unstable. Another factor that may cause or contribute to sticking is the size of the level change. When level changes are very small, the upward or downward force they exert may not be enough to counter the frictional force between the guide 7 and the rod 5. Sticking due to one or both of these factors has been observed in this type of level sensor 10.
The accuracy of measuring level changes is also affected if thermal expansion and contraction cause any changes in the size of the rod 5 or the mounting system 3. When the sensor is mounted at the fill hole or in the top portion of the tank 1, it is subjected to the temperature changes occurring in the vapor space above the liquid surface; these are generally more extreme than the temperature changes that occur below the surface, and as a result the amount of expansion and contraction in the rod 5 and/or mounting system 3 and/or fill tube 2 can be significant. Measurement errors are largest when the level of the liquid in the tank 1 is low and there is a considerable distance between the float 6 and the connector 11 at the fill tube 2 (FIG. 1a) or between the float 6 and the connector 12 at the mount 3 (FIG. 1b). Compensating for thermally induced changes in the length of the equipment (fill tube 2, rod 5 and mounting system 3) by measuring the temperature changes in the vapor space has been attempted.
In U.S. Pat. No. 4,852,054, Mastandrea describes a float/level-sensing system that uses the measurement concept illustrated in FIG. 1. This level sensor 10 is part of a larger system for detecting leaks in underground storage tanks 1. A linear variable displacement transducer (LVDT) 8/9, which is an off-the-shelf, commercially available inductive sensor (in this case manufactured by Schaevitz, Inc.), is used to measure the vertical displacement of a float 6. The LVDT 8/9 is an electromechanical device that consists of a coil assembly 9 and a separate, movable core 8. The coil assembly 9 produces an electrical output proportional to the displacement of the core 8 as this core passes vertically through the coils. The LVDT 8/9 and the float-and-rod subsystem 4 are mounted in a cylindrical tube, which is suspended from the top of the tank. One of the problems encountered with the Mastandrea level-sensing system is that if the diameter of the float 6 is too large with respect to the diameter of this tube, the effects of surface tension will prevent the float 6 from accurately tracking the displacement of the surface. Other problems include the susceptibility of the rod 5 to sticking and the fact that thermal changes in the vapor space can cause the equipment to expand and contract. These are both sources of measurement error, as discussed in the preceding two paragraphs.